The integration of hundreds of millions of circuit elements, such as transistors, on a single integrated circuit necessitates further dramatic scaling down or micro-miniaturization of the physical dimensions of circuit elements, including interconnection structures. Micro-miniaturization has engendered a dramatic increase in transistor engineering complexity, such as the inclusion of lightly doped drain structures, multiple implants for source/drain regions, silicidation of gates and source/drains, and multiple sidewall spacers, for example.
The drive for high performance requires high speed operation of microelectronic components requiring high drive currents in addition to low leakage, i.e., low off-state current, to reduce power consumption. Typically, the structural and doping parameters tending to provide a desired increase in drive current adversely impact leakage current.
Metal gate electrodes have evolved for improving the drive current by reducing polysilicon depletion. However, simply replacing polysilicon gate electrodes with metal gate electrodes may engender issues in forming the metal gate electrode prior to high temperature annealing to activate the source/drain implants, as at a temperature in excess of 900° C. This fabrication technique may degrade the metal gate electrode or cause interaction with the gate dielectric, thereby adversely impacting transistor performance.
Replacement gate techniques have been developed to address problems attendant upon substituting metal gate electrodes for polysilicon gate electrodes. For example, a polysilicon dummy gate is used during initial processing until high temperature annealing to activate source/drain implants has been implemented. Subsequently, the polysilicon is removed and replaced with a metal gate.
Replacement gate techniques have conventionally employed a wet etch approach, for example using tetramethylammonium hydroxide (TMAH), to remove the polysilicon. However, wet etching the polysilicon has been found to be unreliable. TMAH is surface sensitive. Therefore, the replacement gate must be polished down to the polysilicon prior to etching the polysilicon with TMAH. If the polishing step leaves behind any residue, the TMAH will fail to completely remove the polyslilicon. To achieve an effective polishing of the surface, the gate spacers must first be thinned laterally (a spacer shaper step), which may contaminate the polysilicon and interfere with the removal thereof. Further, the wet etch of the polysilicon forms a gap with vertical sidewalls and, thus, a high aspect ratio, which renders difficult the subsequent metal deposition and metal fill in the gap.
A need therefore exists for methodology enabling complete removal of the polysilicon dummy gate, eliminating the need for a spacer shaper step, and forming a gap having a low aspect ratio.